The glutamine synthetase of E. coli has been reported to exhibit univalent cation effects. Since this enzyme plays a central role in nitrogen metabolism, we are currently making a comparative study of such salt effects on the glutamine synthetases of E. coli and of other organisms. In addition, we have observed that the mechanism of cation activation of tryptophanase results from the K ion induced binding of the pyridoxal-5'-phosphate moiety to the active site of the enzyme. We are now considering extending these studies to determine if this mode of activation is common among other pyridoxal-5'-phosphate requiring enzymes involved in amino acid catabolism.